A conventional traveling wave ultrasonic motor comprises an annular stator and an annular rotor which are pressed together in contacting fashion, and an annular piezoelectric element bonded to a back surface of the stator, wherein in operation, the piezoelectric element is excited to generate a traveling wave on a press-contact face of the stator, and an elliptical motion at contact points is converted into a rotational motion of the rotor (reference is made to Takashi Kenjo and Toshiiku Sashida, "An Introduction to Ultrasonic Motors", Sogo Denshi Shuppan, 1991).
Ultrasonic motors of this type, compared with electromagnetic motors, have the advantage of being able to achieve compact and light weight construction owing to the lack of need for magnetic circuits such as a coil winding and an iron core and yet to produce high torque at slow rotational speed, and are commercially implemented in the fields of camera lens rotating mechanisms, robot actuators etc.
Since the conventional ultrasonic motor is of the type that the rotor is frictionally driven by generating a traveling wave on the surface of the stator, in order to generate a traveling wave of large amplitude a piezoelectric element formed of a piezoelectric material having sharp resonance is driven to resonate at its resonant frequency. Consequently, when it is desired to control the rotational speed of the rotor in a variable manner, the traveling wave drive frequency or drive voltage must be varied, but when the drive frequency or drive voltage of the piezoelectric element is varied, the vibration output of the piezoelectric element will drop abruptly or the torque will change. As a result, variable control of the rotational speed is extremely difficult to achieve in the conventional ultrasonic motor and, in practice, the use of this type of ultrasonic motor is limited to applications where on-off control of a constant rpm output is performed, just like typical DC motors.
In view of the above situation, a double drive-type ultrasonic motor has been proposed in which piezoelectric elements are attached to both the stator and the rotor so that the rotational speed of the rotor can be controlled in a continuously variable manner by the interaction between the traveling wave on the stator and the traveling wave on the rotor (Japanese Unexamined Patent Publication JP-A 2-179281 (1990) Japanese Examined Patent Publication JP-B2 2663164).
In this double drive-type ultrasonic motor, since the rotor is also provided with a piezoelectric element, power must be supplied to the rotating piezoelectric element by a suitable means.
In the ultrasonic motor described in JP-A 2-179281, the rotor and stator are housed within a case, an annular conductive plate is disposed on the upper surface of a rotor-side elastic member (the surface facing the interior surface of the upper wall portion of the case), and a conductive brush which contacts the conductive plate in rubbing fashion is fixed to the interior surface of the case so as to face the conductive plate, wherein a signal of a given frequency is supplied to the rotatable piezoelectric element at the rotor side via a power feed unit consisting of the conductive brush and the conductive plate.
In this ultrasonic motor, however, since the power feed unit is disposed between the rotor-side elastic member and the interior surface of the upper wall portion of the case, a particular space capable of accommodating the power feed unit has to be provided between the rotor-side elastic member and the interior surface of the upper wall portion of the case, which leads to the problem that the size of the motor inevitably increases in the axial direction.
Furthermore, since the ultrasonic motor is configured so that the conductive brush contacts the conductive plate in rubbing fashion with the conductive brush fixed to the interior surface of the cover and the conductive plate to the rotor-side elastic member, the cover and the motor mechanism must be matched (aligned) against each other, but the task of matching is extremely difficult, resulting in the problem that the manufacturing increase.
On the other hand, for the traveling wave ultrasonic motor first described, a power feed method in which a rotary transformer or a combination of a slip ring and a brush is used is proposed as a method applicable to an ultrasonic motor where the piezoelectric element is attached to the rotor, not to the stator (Japanese Unexamined Patent Publication JP-A 4-71371 (1992)). However, the detailed configuration using the rotary transformer is not presented, though the configuration using the slip ring is described in detail therein.